This invention relates generally to microfabrication technology and to the formation of three-dimensional microstructures. The invention has particular applicability to microstructures for transmitting electromagnetic energy, such as coaxial transmission element microstructures, and to methods of forming such microstructures by a sequential build process.
The formation of three-dimensional microstructures by sequential build processes have been described, for example, in U.S. Pat. No. 7,012,489, to Sherrer et al. With reference to FIG. 1, the '489 patent discloses a coaxial transmission line microstructure 2 formed by a sequential build process. The microstructure is formed on a substrate 4, and includes an outer conductor 6, a center conductor 8 and one or more dielectric support members 10 which support the center conductor. The outer conductor includes a conductive base layer 12 forming a lower wall, conductive layers 14, 16 and 18 forming sidewalls, and conductive layer 20 forming an upper wall of the outer conductor. The volume 22 between the inner and outer conductors is air or vacuous, formed by removal of a sacrificial material from the structure which previously filled such volume.
When fabricating microstructures of different materials, for example, suspended microstructures such as the center conductor in the microstructure of the '489 patent, problems can arise due to insufficient adhesion between structural elements, particularly when the elements are formed of different materials. For example, materials useful in forming the dielectric support members may exhibit poor adhesion to the metal materials of the outer conductor and center conductor. As a result of this poor adhesion, the dielectric support members can become detached from either or both of the outer and center conductors, this notwithstanding the dielectric support member being embedded at one end in the outer conductor sidewall. Such detachment can prove particularly problematic when the device is subjected to vibration or other forces in manufacture and post-manufacture during normal operation of the device. The device may, for example, be subjected to extreme forces if used in a high-velocity vehicle such as an aircraft. As a result of such detachment, the transmission performance of the coaxial structure may become degraded and the device may be rendered inoperable.
There is thus a need in the art for improved three-dimensional microstructures and for their methods of formation which would address problems associated with the state of the art.